President Reagan struck an upbeat tone about a prospective treaty on medium- and short-range nuclear missiles yesterday, praising the Soviets for accepting recent U.S. arms proposals and confirming that the United States is offering concessions in response.

"There is still much to do in Geneva, but I'm heartened that the climate is now receptive to an historic proposal of this type," Reagan told a government superconductor conference here.

White House spokesman Marlin Fitzwater, meanwhile, confirmed State Department reports that Secretary of State George Shultz will meet with Soviet Foreign Minister Eduard Shevardnadze to discuss arms control and other issues when he comes to his country for the U.N. General Assembly meeting in mid-September.

Although the last such meeting in April disappointed both sides and resulted in little arms control progress, Fitzwater said the commitment for a new meeting is "an optimistic sign" for an eventual summit between Reagan and Soviet leader Mikhail Gorbachev at which a treaty could be signed.

"We are pleased that the Soviets have now expressed support" for a 1981 U.S. proposal that medium-range missiles be eliminated and a more recent U.S. offer to eliminate all shorter-range missiles, Reagan said.

Soviet acceptance of the first proposal came after the United States formally backed away in response to Soviet demands at a summit meeting in Iceland last year. U.S. officials and independent experts said that Gorbachev was the first leader to suggest elimination of short-range missiles.

Reagan and Fitzwater said U.S. negotiators offered concessions in Geneva barring conversion of existing missiles into weapons that would not be covered by the agreement, and easing measures to verify U.S. and Soviet compliance, as previously reported.

Asked why the White House was publicly announcing its position at confidential Geneva negotiations, Fitzwater replied, "This ain't chopped liver, man, this is on the table . . . . It's important that {Reagan} simply let the American people know that we are continuing to make progress."

Fitzwater denied Reagan is concerned about losing a public-relations battle with Gorbachev, whom some Europeans believe is responsible for recent negotiating breakthroughs.

Reagan also said he hopes the Soviets will respond to a draft treaty on strategic weapons introduced by the United States this spring with a detailed draft treaty. His remarks, repeated by State Department spokesman Charles E. Redman, were apparently sparked by fresh indications that the Soviets will introduce a draft treaty on strategic weapons this week.

Several officials also noted that the Soviets had requested a formal "plenary" meeting of negotiators in Geneva today, raising expectations that they will make a major announcement about space weapons, the third major arms-control issue in dispute.